Barrier coatings are widely used in packaging materials to prevent the passage of a permeant molecule or composition, especially to prevent contact between the contents of a package and the permeant. Improving barrier properties is an important goal for manufacturers of films sold for containment of products such as foods, cosmetics, agricultural chemicals, and pharmaceuticals. Injurious permeant chemicals of interest include oxygen, carbon dioxide, water vapor, aromatic and aliphatic hydrocarbons, manufacturing residues, off odors, off flavors, smoke, pesticides, toxic materials, and environmental contaminants and pollutants. Use of typical plastic materials is desirable because they are inexpensive. Plastics, however, are oxygen-permeable to such a degree that the amount of oxygen permeated is much higher than a metal or glass material as used in canning or bottling, or foil used with paper for packaging soup mixes and dry snacks, all of which have an oxygen permeability of substantially zero.
Barrier coatings can also serve to (a) keep a gas inside a package—e.g., a gas used in modified atmosphere packaging, or helium kept inside balloons; (b) keep moisture inside a package so that the contents do not dehydrate; or (c) keep a perfume inside a package, where perfumes can be expensive components. In all these cases, the barrier is maintaining the packaged contents.
Barrier properties arise from both the structure and the composition of the material. The order of the structure, i.e., the crystallinity or the amorphous nature of the material, the existence of layers or coatings can affect barrier properties. The barrier property of many materials can be increased by using liquid crystal or self-ordering molecular technology, by axially orienting materials such as an ethylene vinyl alcohol film, or by biaxially orienting nylon films and by using other useful structures. Internal polymeric structure can be crystallized or ordered in a way to increase the resistance to permeation of a permeant. A material may be selected for a plastic or paper packaging coating that prevents absorption of a permeant onto the barrier surface, and a material may be selected to prevent the transport of the permeant through the barrier. Generally, permeation is concentration and temperature dependent. Permeability is also a function of pressure, where a gradient exists between atmospheric pressure and the structure surrounded by a coated film, etc., e.g., balloons (positive pressure) and vacuum packaging (negative pressure).
Permeation through a polymeric coating is believed to be a multi-step event. First, collision of the permeant molecule, such as oxygen, with the polymer is followed by sorption into the polymer. The permeant migrates through the polymer matrix along a random path, and finally the permeant desorbs from the polymer. The process reaches equilibrium (chemical concentration and pressure) on both sides of the coating. Permeability of a typical molecule through a packaging film is a function of diffusion rate and solubility of the molecule. The diffusion rate measures how rapidly transport of the molecule occurs through the film, and it relates to the ease with which a permeant molecule moves within a polymer. Solubility relates to the concentration or total amount of permeant that may be present in the film. Diffusion and solubility are important measurements of barrier coating performance. Transfer of vapors through packaging films may occur by capillary flow or activated diffusion. Capillary flow involves small molecules permeating through pinholes or microscopic channels of porous media, which is generally an undesirable feature of a barrier coating. Activated diffusion entails solubilization of a permeant into an effectively non-porous medium, diffusion through the film under a concentration gradient, and release from a transverse surface at a lower concentration. Several factors determine the ability of a permeant molecule to permeate through a barrier coating, including size, shape, and chemical nature of the permeant, physical and chemical properties of the polymer, and interactions between the permeant and the polymer.
Various transparent plastic materials having unsatisfactory gas barrier properties are known. Films consisting of a thermoplastic resin, oriented films of polypropylene, polyester, polyamide or the like typically have excellent mechanical properties, heat resistance, transparency and the like and are widely used as packaging materials. However, when these films are used for packaging foods or other goods, they are unsatisfactory for high barrier requirements to oxygen and other gases. Typical barrier materials are a single layer of polymer, a bilayer co-extruded or laminated polymer film, a coated monolayer, or a bilayer or multilayer film having one or more coatings on a surface or both surfaces. The most widely used barrier polymers for food packaging are ethylene-vinyl alcohol copolymers (“EVOH”), ethylene vinyl acetate copolymers (“EVA”), and polyvinylidene chloride terpolymers (“PVDC”), which offer some resistance to permeation of gases, flavors, aromas, and solvents. PVDC also offers some resistance to moisture. EVOH copolymer resins are available in a wide variety of grades having varying ethylene concentrations. As the EVOH content is increased relative to the polyethylene content, the barrier properties to gases, flavors, and solvents increase. EVOH resins are commonly used in coextrusions or laminations with polyolefins such as polyethylene and polypropylene as structural and/or sealant layers, and with nylon, polyethylene terephthalate (“PET”), poly(lactic acid) (“PLA”), or polyhydroxyalkanoate (“PHA”) as structural layers. PVDC emulsions are applied as micron-thick rotogravure coatings to various base film structures such as PET, nylon, polypropylene, poly(lactic acid) (“PLA”), or polyhydroxyalkanoate (“PHA”). Other barrier technologies include metallization with thin coatings of aluminum to various base film structures using vacuum deposition. Moderate barrier polymer materials such as monolayer polyethylene terephthalate, polymethyl pentene, and polyvinyl chloride (“PVC”) films are commercially available.
Still other barrier films have been achieved with very thin plasma vapor depositions of oxides of silicon or aluminum (several nanometers thick) on base films and molded polymer structures.
Another barrier technology involves the use of oxygen absorbers or scavengers that are used in polymeric coatings or in bulk polymer materials. Metallic reducing agents such as ferrous compounds and powdered oxide or metallic platinum can be incorporated into barrier systems, which scavenge oxygen by converting it into a stable oxide within the film. Non-metallic oxygen scavengers have also been developed and are intended to alleviate problems associated with metal or metallic tastes or odors. Such systems include compounds including ascorbic acid and various salts and organometallic compounds that have a natural affinity for oxygen. Such molecules absorb oxygen molecules into the interior polymer chemical structure removing oxygen from the internal or enclosed space of packaging materials. Such materials are expensive and, in some cases, the presence of hazardous antioxidants chemicals limits their application.
Another method for imparting gas barrier properties includes dispersing an inorganic material in a resin. Micron-thin polymeric coatings incorporate nano-scale particulate dispersions of clays, such as montmorillonite, hectorite, sodium terasililic mica, sodium taeniolite, and vermiculite into various water-solubilized or emulsified polymers. For example, montmorillonite, hectorite, sodium terasililic mica, or sodium taeniolite may be blended into polyvinyl alcohol. Similarly, polyvinyl alcohol/poly(acrylic acid) blends with these clays are known. In order to prevent clay or vermiculite particles from aggregating or precipitating from solution while mixed with such polymers, it must be extensively pre-treated with, for example, acetic acid or glycine. Still, it is difficult to maintain vermiculite particles in suspension.
Finally, attempts to create barrier by direct addition of various clay particles in extruded and blown thermoplastic films and molded articles are common, but have only modest improvements in barrier impermeability versus orders of magnitude improvement using the aforementioned clay-containing coatings.